Heat transfer device useful for extruders

ABSTRACT

Heat transfer devices useful for extruders. Jacket sections defining inwardly directed ribs are heat shrunk about extruder barrels. By appropriate sizing, the stresses produced exceed the yield stresses causing metal to self-seal any nicks occurring and to expand adjacent base portions tighter against the inner member or barrel. Arelatively hard barrel concentrates the yielding to the metal of the jacket. Heat shrunk ribs provide good heat transfer paths to electric heaters secured about the exterior of the jackets, as well as defining fluid passages. Free flanges joined by narrow root sections enable joining of adjacent jacket sections by welds with immunity to disturbance in the presence of differential expansion of the assembly. Intercommunication of annular compartments is provided by interruption in the ribs or by covered grooves formed through the free side of the jacket wall.

United States Patent 1 Schott, Jr.

1 1' Apr. 17,1973

[54] HEAT TRANSFER DEVICE USEFUL [73] Assignee: Gloucester EngineeringCo., Inc.,

Gloucester, Mass.

[22] Filed: Dec. 30, 1970 [21] App]. No.: 102,627

Primary Examiner-Charles Sukalo Attorney-John Noel Williams ABSTRACTHeat transfer devices useful for extruders. Jacket sections defininginwardly directed ribs are heat shrunk about extruder barrels. Byappropriate sizing, the stresses produced exceed the yield stressescausing metal to self-seal any nicks occurring and to expand adjacentbase portions tighter against the inner member or barrel. Arelativelyhard barrel concentrates the yielding to the metal of the jacket. Heatshrunk ribs provide good heat transfer paths to electric heaters securedabout the exterior of the jackets, as well as defining fluid passages.Free flanges joined by narrow root sections enable: joining of adjacentjacket sections by welds with immunity to disturbance in the presence ofdifferential expansion of the assembly. lntercommunication of annularcompartments is provided by interruption in the ribs or by coveredgrooves formed through the free side of the jacket wall.

- 6 Claims, 10 Drawing Figures PATENTE D APR 1 7 i975 SHEET 2 0F 6 FIG 4PATENTED APR 1 71975 sum u or 6 FIGS PATENTEBAPR 1 7197s SEiEEI 5 UF 6HEAT TRANSFER DEVICE USEFUL FOR EXTRUDIERS This invention concerns heattransfer assemblies of the fluid compartment type useful for plasticextruders and other equipment where cooling and heating are required.

In plastic extruders a plastic compound is subjected to mechanicalworking effects and heat, to render it fluid and to force it through anorifice under substantial pressure. Various degrees of heating andcooling of an extruder at various points along its length are desiredfor start-up and running conditions and for heating the extruder to theextent that the extruder screw may be removed. Variability of thetemperature along the extruder length over a wide range will permit agiven extruder design to be employed with various plastics materialswhere physical properties and extruding con ditions vary.

One object of the invention is to provide improved heat transferassemblies of the fluid compartment type.

Another object of the invention is to provide improved extruders whichpermit desired temperature control.

Other objects include providing: reliable and lowcost, leak-proof sealsfor coolingjackets which can perform over a wide operating temperaturerange; simple jacket constructions which facilitate both efficientcooling and heating and are practical to construct and operate; jacketconstructions which absorb distortions of the jackets and preventwarping of the internal member; and jacket constructions of a standardform useful over a wide range of applications.

In one aspect the invention features a heat transfer assembly useful forplastics extruders having a hollow metal outer jacket disposed about aninternal metal member and defining therebetween a confining volume forheat transfer liquid. The jacket member has an in tegral liquid-sealingcircular rib member protruding toward and deformed by interferenceshrink fit against and in liquid-sealing relation with a correspondingouter surface ofthe internal member. The unassembled internal diameterof the rib member is less than the outer diameter of the internal memberwhen both are at the same temperature and the rib member is shrunk fitupon the internal member by cooling of the jacket from a temperaturehotter than that of the internal member. In preferred embodiments theinternal member is an extruder barrel and the jacket defines a coolingpassage about the outer periphery of the barrel; the outer surface ofthe internal member is constructed of metal harder than that of thejacket; the outer surface of the internal member is smooth and thejacket includes additional rib members defining a path for the heattransfer liquid, these also being deformed by interference shrink litwith the internal member; and the assembly has at least one annularelectric resistance band heater disposed around the jacket and arrangedto transfer heat to the internal member through the shrink-fit ribmembers.

According to another aspect of the invention a portion of the projectionofa rib member is in permanently deformed condition as a result ofstresses in excess of the elastic limit produced from the interferencefit, preferably the projection extending from a base, and as a result ofdislocation of metal of the projection, the

base being extended toward and pressed against the internal member,contributing to the effectiveness of the seal. The metal of the rib canbe formed into and sealed against surface irregularities occurring inthe area of the seal.

According to another aspect of the invention the jacket is formed by aplurality of jacket sections with internal ribs of unassembled internaldiameter less than corresponding portions of the barrel or internalmember and the jacket sections are shrunk fit onto the internal memberor extruder barrel with the ribs pressing tightly upon the outer surfaceof the barrel in heat transferring relation therewith and the heaters,in the form of annular bands surrounding portions of the jacket, arearranged to transfer heat through the ribs to the extruder. Adjacentsections of the jacket have mating radial flange portions atcorresponding ends which are welded together and each of which is joinedto its jacket section through a root portion of thickness less than thegeneral wall thickness of the jacket, whereby differential expansion ofadjacent sections of the jacket may be accommodated with bending of theflanges and without disturbance of the relationship of the ribs upon thebarrel.

In another aspect annular compartments are defined between adjacent ribsof the jacket and liquid passages are provided connecting selectedcompartments. In one form the passages comprise openings in adjacentribs spaced circumferentially from each other to define a circuitouspath for coolant flowing successively through the compartments while inanother form grooves machined in the outer wall of the jacket permitcross over of any numberof compartments, openings being provided intothe groove from the selected compartments and a cover plate applied overthe groove.

Other objects, features and advantages will become apparent from thefollowing description of a preferred embodiment of the invention, takentogether with the attached drawings thereof, in which:

FIG. I is a side elevation, partially in cross-section and partially indiagrammatic form, of an extruder constructed in accordance with theinvention;

FIG. 2 is a perspective view of an extruder jacket of FIG. 1 before itis assembled around the barrel, with a portion of the jacket brokenaway;

FIG. 3 is a cross-sectional view of portions of the jacket before thejacket is assembled around the barrel;

FIG. 4 is a diagrammatic view of the flow paths established by thejacket around the barrel;

FIG. 5 is an enlarged crosssectional view of the ribs and endconstruction ofajacket section with an indication also of the relativesize of the barrel;

FIG. 6 is an enlarged cross-sectional view of portions of the jacket andbarrel adjacent an end seal; and

FIG. 7 is an enlarged cross-sectional view of a portion of the end sealadjacent a surface irregularity of the barrel;

FIG. 8 is a diagrammatic cross-sectional view,

FIG. 9 is a partially cut away perspective view and FIG. 10 is a detailof another preferred embodiment of the invention.

Referring to FIG. 1 an extruder comprises a hopper 12 arranged to feedplastic pellets to a barrel 14. In the barrel a threaded screw 16 isrotated by a motor 18, to work the plastic from solid to molten form,and propel the molten plastic toward barrel end 20. At end 20 the barrelhas an orifice 22 communicating with a die or other plastic formingapparatus (not shown).

The major thickness of the barrel is typically of 4140 Alloy steel witha durable X aloy(trademark of Xaloy Incorporated) lining at the bore ofthe barrel.

The portion of barrel l4 beyond hopper 12 is divided into a number ofzones, each having an annular cooling jacket 34, 36, 38, 40 and 42 (tobe discussed in greater detail below) surrounding the barrel. Eachjacket is separately operable and arranged to receive a cooling fluid,for example, ethylene glycol, from a fluid heat transfer system 44through pipes 46, to permit circulation of the fluid to cool barrel 14,and to return the fluid to heat transfer system 44 through pipes 48.Annular electric band heaters 24 are disposed about the jackets, thetemperature of each of which is controlled at heat control panel 28 overleads 30 and 32. The extruder is further provided with a sixth coolingjacket 52, and a sixth band heater 50, each of which is separatelyoperable, around the portion of barrel 14 which surrounds the driveshaft 54 of screw 16 adjacent motor 18.

Cooling jackets 34, 36, 38, 40, 42 and 52 are substantially identicaland for simplicity only end jacket 42, which incorporates the differentfeatures of all of the jackets, will be discussed in detail. As can bestbe seen in FIGS. 2, 3 and 4jacket 42, which is constructed of low carbonsteel, is substantially tubular in shape, has a smooth outer surface 56and has its inner surface machined to provide a plurality of circularribs 60 (54 inch wide, w, approximately 0.3 inch deep, d, and spacedapart by 2 inch,s,)at the inner periphery, projecting radially inward.Each rib is interrupted, providing cross spaces 62 between the ends 64of ribs 60. Spaces 62 on adjacent ribs 60 are diametrically oppositeeach other.

At its outer -or sealing end 66 (FIGS. 3,4 and jacket 42 has a rib 61 ofthe same radius R;, as rib 60, and projecting 0.004 inches, d,, radiallyinward from rib 61 is a sealing land 68 (0.062 inches wide,w,). Spaced0.50 inches,L, from'the inner end 70 (left end, FIG. 5) ofjacket 42 is amiddle joint 71 in which a rib 72, 0.25 inches wide, projects 0.3 inchesradially inward to alignment with the ribs 60,61. In this joint regionouter surface 56 has annular groove 74, (0.125 inches wide and 0.625inches deep radially), cut to provide a deformable flange section 76having a root portion 77 (approximately 0.3 inches thick)and curvedcurved (V4th circle) annular welding groove 78 cut in the outer cornerthereof. The inner corner is chamfered as shown. The end of this flangeis butted against a corresponding flange of the next adjacent jacket 40,the welding grooves forming together a semicircular groove in which awelding bead 79 is laid.

Ribs 60, 61 and 72 define annular cooling fluid passages 84 between thejackets and barrel l4 and spaces 62 define longitudinally extendingpassages which connect adjacent passages 84. Jackets 36, 38 and 40 havemiddle joints 7] at both of their ends, while jackets 34 and 42 have onesealing end 66 at one end and a middle joint 71 at the opposite end.Jacket 52 has two sealing ends 66.

Each jacket section has a fluid inlet passage (FIG. 4) communicatingwith a pipe 46 and the passage 84 between the nearest two ribs to oneend of the jacket and a fluid outlet passage communicating with a pipe48 and the passage 84 between the nearest two ribs to the other end ofthe same jacket.

The inner surfaces of ribs 60, 61 and 72 lie at a radius R; from axis ofsymmetry A (i.e. 4.931 inch) which is smaller than the outer radius R ofbarrel l4 i.e. 5 inches. For assembly, jacket sections 34, 36, 38, 40,42 and 52 are heated in an oven to a temperature higher than thetemperature of barrel 14, eg to 600F, until the metal from which thejackets are constructed expands sufficiently for the effective innerradius of the jackets to become greater than R The jackets are thenslipped over barrel 14 in the proper sequence and moved into theirrespective positions and permitted to cool with the ends of adjacentjackets firmly in place against each other. As the jackets cool theybecome interference or shrunk fit (see later discussion for details ofthis fit) around barrel 14 and after they cool, adjacent grooves 78 arewelded together and heaters 24 and 52, leads 30 and 32, pipes 46 and 48and motor 18 assembled in place. After the jackets have cooled, theinner surfaces 80 of ribs 60, 61 and 83 are in tight contact with barrel14 under all operating conditions.

Referring now to FIGS. 6 and 7, the effects of the shrunk fit on theprojection 68 at end seal 66 are illustrated. Since the 4140 alloy steelfrom which barrel 14 is constructed is harder than the low carbon steelfrom which the jackets are constructed, substantially all permanentdeformation caused by the stresses resulting from the interference fitoccur in the jacket. I have found that the stress generated between thebarrel 14 and the sealing land 68 is approximately 8,000 pounds per inchof circumference and that under such conditions the metal in lands 68deforms beyond the yield point of the metal. For example, if the outersurface of barrel 14 has an irregularity 86 (FIG. 7) therein such as anick in the outer surface of the barrel, metal from a sealing land flowsinto and seals against the irregularity 86. Furthermore, the stress towhich the sealing lands are subjected is sufficient to cause plasticflow (of the steel) to occur in rib 61. In FIG. 5, at the right handside, dotted lines 80a and 82a show the relative locations of surfaces80 and 82 before assembly, while lines 80b and 82b show their respectivelocations after jacket 42 has partially cooled, assuming no barrel inplace. Lines 800 and 820 diagrammatically illustrate the location of thesurfaces at the same partially cooled temperature with the barrel inplace, and the projection and rib shrunk fit around barrel 14. Land 68has been deformed outwardly toward rib 60 and plastic flow has occurred,as along lines 88, with the result that except for the region 90immediately adjacent land 68 the metal in rib 60 is deformed inwardlyfrom its unstressed position toward the surface of barrel 14. The stresseventually drops below the yield point after the deformed portion of rib60 is raised sufficiently for surface 800 to come into sealingengagement with barrel 14 over a width greater than the width of sealingland 68, this raising contributes positively to the effectiveness of thepurely metal-to-metal seal, and also it introduces considerableresistance to relative sliding of the jacket on the barrel duringdifferential expansion, and so protects the projection 68 from beingsheared off.

In operation heaters 24 and motor 18 are started and plastic pelletsdeposited in hopper 12. Since heaters 24 are in direct contact with theouter surface 56 of the cooling jacket, the heat which they generate israpidly conducted to barrel 143 by ribs 60 and 72 which are in tegralwith the jackets and shrunk fit upon the barrel, guaranteeing a goodheat conductivity path. The pellets enter barrel l4 and are driven andworked, as they are heated and melted, in the direction of orifice 22.As the working of the plastic continues and as the temperature of themolten plastic increases with its movement toward end 20, the mechanicalworking of the screw 16 tends to cause undesirable increases in thetemperature of the plastic in barrel in some or all of the zones andrequires cooling.

(It is extremely important in the extruding process that, for a givenmaterial and given settings of the extrusion apparatus and dies, thatthe temperature characteristics of the apparatus remain as constant asis possible to maintain the desired viscosity of the molten materialbeing processed to insure uniformity in the finished product, forexample, thin plastic sheets. in addition, with some vinyls, degradationoccurs if the temperature becomes excessive. Thus the cooling jacketsare provided to permit localized cooling of the barrel for each zonewhen conditions warrant.) An attendant may regulate the flow of thecooling fluid from heat transfer system 44 to each of the jacketsections through pipes 46 in order to maintain acceptable temperaturewithin the particular zone surrounded by a jacket.

The fluid enters the jackets at the inlet and first flowscircumferentially in passages 84 around and in contact with barrel 114,then longitudinally forward through space 62 toward the outlet end, andis urged circumferentially in reverse direction around barrel 14 inpassages 84 by ribs 60. Circumferential flow alternates between flow inthe upward and downward directions until the fluid reaches the outletand is returned to system 44 by pipes 48.

The sealing lands 68, at the outside ends of jackets 34 and 42 and ofjacket 50, which have deformed beyond their yield point, together withsufficiently deformed portions of ribs 60, effectively prevent anyleakage of the fluid even if the barrel 14 has irregularities (such asthat shown in FIG. 7) in its outer surface, caused, for example, bypeening required to remove warps from barrel 14 after it has beenformed, since the low carbon steel in lands 68, while under high stress,has deformed into the filled them. Similarly the welds between adjacentjackets in grooves 78 around barrel l4 effectively seal the middlejoints 711 to prevent fluid leakage to the atmosphere at normaloperating pressures of approximately 60 p.s.i. When differentialexpansion or contraction of adjacent jackets or welding distortionsoccur, the flange 76 at the middle joint flex to take up this effectwithout forcing the ribs to shift on the barrel.

Thus, the jacket of the invention provides a seal which is extremelyreliable, but at the same time simple and inexpensive to construct.Furthermore, the grooves cut in the inner surface of the jackets todefine the ribs 60 and lands 68, because of their cylindrical, ratherthan the more conventional helical, configuration may be quickly andinexpensively cut by employing a right angle milling head or an internalshaper, rather than requiring alathe with which the direction of cuttingmust be reversed for each turn if, for example, a helical groove were tobe cut. Constructing the overall jacket from a number of shortersections simplifies both assembly of the jacket (since the sections aremore easily handled than a largerjacket) and the heating apparatusrequired to expand the sections sufficiently to fit them over barrel 14.

Referring to H68. 8, 9 and 10, another preferred embodiment is shown inwhich the ribs form continuous hoops and the compartments defined by theribs are connected through cross passages provided in the outer wall d3of the jacket member. To this end, a groove is machined in the outersurface of the jacket axially along the jacket, between any two compart'ments desired to be connected. Holes 92 are then drilled to connect thegroove with these compartments. Then acover plate 94 is welded in placeover the groove, completing the cross-over passage. By this means verycomplex flow patterns can be established and can be changed from time totime if desired by opening some passages and closing others.

Other embodiments will occur to those skilled in the v art and arewithin the following claims.

What is claimed is:

1. in a heat transfer assembly comprising a hollow, rigid metal externaljacket member of circular crosssection disposed about a rigid internalmetal member of similar cross-section and defining therebetween anannular confining volume for heat transfer liquid, one of said internaland external members having axially spaced-apart annular ribs extendingradially into con tact with the other member, each of said members beingpreformed, said members being presized for heat-shrink fitting together,the preformed radial dimension of said ribs being in excess of the spacebetween said members, and in said assembly said ribs residing undersubstantial radial compression, there being, in reaction, substantialhoop tension stress in said outer member, and radial compression of saidinner member in the region of said annular ribs, the improvement whereinthe circular surface: of said member engaged by one of said ribs has asurface discontinuity, said rib having preformed on its engaging surfacea land of narrow axial dimension relative to the axial dimension of saidrib, and a radial extent greater than that of said rib, said land beingpresized for compressional stressing beyond its metal yield point byheat-shrink fitting of said members together, in said assemblysaid landlying over said surface discontinuity and in a state deformed beyond itselastic limit, the substance of said land effectively sealing againstsaid] surface discontinuity.

2. The heat transfer assembly of claim 1 wherein said internal metalmember is substantially cylindrical, said external member has integral,radially inwardly extending ribs, and said land extends integrally,radially inwardly from its respective rib.

3 The heat transfer assembly of claim 1 wherein said external jacketmember comprises a plurality of axially contiguous sections, eachdefining with respective ribs an annular confining volume portion forheat transfer liquid, said sections joined by radially outwardlyextending circular walls joined together at an outer radius, said wallshaving axially displaceable portions inwardly thereof, said joined wallsforming a liquidtight expansion joint permitting one of said sections tothermally axially contract without displacing the other of said sectionsin the axial direction, whereby said land and ribs are protected fromdetrimental axial shearing stress.

4. In a heat transfer assembly comprising a hollow, rigid metal externaljacket member of circular crosssection disposed about a rigid internalmetal member of similar cross-section and defining therebetween anannular confining volume for heat transfer liquid,one of said internaland external members having axially spaced-apart annular ribs extendingradially into contact with the other member, each of said members beingpreformed, said members being presized for heat-shrink fitting together,the preformed radial dimension of said ribs being in excess of the spacebetween said members, and in said assembly said ribs residing undersubstantial radial compression, there being, in reaction, substantialhoop tension stress in each outer member, and radial compression of saidinner member in the region of said annular ribs, the improvement whereinsaid external jacket member comprises a plurality of axially contiguoussections, each defining with respective ribs an annular confining volumeportion for heat transfer liquid, said sections joined by radially,outwardly extending circular walls joined together at an outer radius,said walls having axially displaceable portions inwardly thereof, saidjoined walls forming a liquid-tight expansion joint permitting one ofsaid sections to thermally axially contract without displacing the otherof said sections in the axial direction whereby said ribs are protectedfrom detrimental axial shearing stress.

5. The heat transfer assembly of claim 4 wherein said internal metalmember is substantially cylindrical, said external member has radiallyinwardly extending ribs, contiguous portions of sections of saidexternal member each having an external annular groove of substantialradial depth spaced a short distance from the respective end of saidsection, the end portion of said section beyond said groove forming saidwall, radially outer portion of the walls of said contiguous sectionsbeing joined together.

6. in a heat transfer assembly comprising a hollow, rigid metal externaljacket member of circular crosssection disposed about a rigid internalmetal member of similar cross-section and defining therebetween anannular confining volume for heat transfer liquid, one of said internaland external members having axially spaced-apart annular ribs extendingradially into contact with the other member, each of said members beingpreformed, said members being presized for heat-shrink fitting together,the preformed radial dimension of said ribs being in excess of the spacebetween said members, and in said assembly said ribs residing undersubstantial radial compression, there being, in reaction, substantialhoop tension stress in said outer member, and radial compression of saidinner member in the region of said annular ribs, the improvement whereinsaid heat transfer assembly is adapted selectively to conduct heat toand from said internal metal member, there being at least one electricalresistance band heater disposed about the exterior of said externalacket member, positioned to conduct heat into the body of said externalmember thence through said rib and the respective heat shrunk joint intosaid internal metal member, and means also for selectively introducingcooling liquid into the annular volume defined between said external andinternal members.

1. In a heat transfer assembly comprising a hollow, rigid metal externaljacket member of circular cross-section disposed about a rigid internalmetal member of similar cross-section and defining therebetween anannular confining volume for heat transfer liquid, one of said internaland exteRnal members having axially spaced-apart annular ribs extendingradially into contact with the other member, each of said members beingpreformed, said members being presized for heat-shrink fitting together,the preformed radial dimension of said ribs being in excess of the spacebetween said members, and in said assembly said ribs residing undersubstantial radial compression, there being, in reaction, substantialhoop tension stress in said outer member, and radial compression of saidinner member in the region of said annular ribs, the improvement whereinthe circular surface of said member engaged by one of said ribs has asurface discontinuity, said rib having preformed on its engaging surfacea land of narrow axial dimension relative to the axial dimension of saidrib, and a radial extent greater than that of said rib, said land beingpresized for compressional stressing beyond its metal yield point byheat-shrink fitting of said members together, in said assembly said landlying over said surface discontinuity and in a state deformed beyond itselastic limit, the substance of said land effectively sealing againstsaid surface discontinuity.
 2. The heat transfer assembly of claim 1wherein said internal metal member is substantially cylindrical, saidexternal member has integral, radially inwardly extending ribs, and saidland extends integrally, radially inwardly from its respective rib. 3.The heat transfer assembly of claim 1 wherein said external jacketmember comprises a plurality of axially contiguous sections, eachdefining with respective ribs an annular confining volume portion forheat transfer liquid, said sections joined by radially outwardlyextending circular walls joined together at an outer radius, said wallshaving axially displaceable portions inwardly thereof, said joined wallsforming a liquid-tight expansion joint permitting one of said sectionsto thermally axially contract without displacing the other of saidsections in the axial direction, whereby said land and ribs areprotected from detrimental axial shearing stress.
 4. In a heat transferassembly comprising a hollow, rigid metal external jacket member ofcircular cross-section disposed about a rigid internal metal member ofsimilar cross-section and defining therebetween an annular confiningvolume for heat transfer liquid, one of said internal and externalmembers having axially spaced-apart annular ribs extending radially intocontact with the other member, each of said members being preformed,said members being presized for heat-shrink fitting together, thepreformed radial dimension of said ribs being in excess of the spacebetween said members, and in said assembly said ribs residing undersubstantial radial compression, there being, in reaction, substantialhoop tension stress in each outer member, and radial compression of saidinner member in the region of said annular ribs, the improvement whereinsaid external jacket member comprises a plurality of axially contiguoussections, each defining with respective ribs an annular confining volumeportion for heat transfer liquid, said sections joined by radially,outwardly extending circular walls joined together at an outer radius,said walls having axially displaceable portions inwardly thereof, saidjoined walls forming a liquid-tight expansion joint permitting one ofsaid sections to thermally axially contract without displacing the otherof said sections in the axial direction whereby said ribs are protectedfrom detrimental axial shearing stress.
 5. The heat transfer assembly ofclaim 4 wherein said internal metal member is substantially cylindrical,said external member has radially inwardly extending ribs, contiguousportions of sections of said external member each having an externalannular groove of substantial radial depth spaced a short distance fromthe respective end of said section, the end portion of said sectionbeyond said groove forming said wall, radially outer portion of thewalls of said contiguous sectIons being joined together.
 6. In a heattransfer assembly comprising a hollow, rigid metal external jacketmember of circular cross-section disposed about a rigid internal metalmember of similar cross-section and defining therebetween an annularconfining volume for heat transfer liquid, one of said internal andexternal members having axially spaced-apart annular ribs extendingradially into contact with the other member, each of said members beingpreformed, said members being presized for heat-shrink fitting together,the preformed radial dimension of said ribs being in excess of the spacebetween said members, and in said assembly said ribs residing undersubstantial radial compression, there being, in reaction, substantialhoop tension stress in said outer member, and radial compression of saidinner member in the region of said annular ribs, the improvement whereinsaid heat transfer assembly is adapted selectively to conduct heat toand from said internal metal member, there being at least one electricalresistance band heater disposed about the exterior of said externaljacket member, positioned to conduct heat into the body of said externalmember thence through said rib and the respective heat shrunk joint intosaid internal metal member, and means also for selectively introducingcooling liquid into the annular volume defined between said external andinternal members.